The invention relates to coated sodium percarbonate particles consisting of a core of sodium percarbonate and a coating which contains sodium carbonate, which constitutes from 1 to 25 wt. % of the sodium percarbonate. The coated sodium percarbonate particles, which also contain a magnesium compound as a coating component, are characterized by high storage stability in the presence of washing powders in a moist and warm environment. The invention further relates to a process for preparing the coated sodium percarbonate particles by applying a mono or multi-layered coating in a fluidized bed. The invention also relates to the use of the coated sodium percarbonate particles as bleaching components in detergent, cleansing agent and bleaching agent formulations.
Sodium percarbonate (2Na.sub.2 CO.sub.3.3H.sub.2 O.sub.2) is used as an active oxygen component in detergents, bleaches and cleansing agents. Due to the unsatisfactory storage stability of sodium percarbonate in a warm and moist environment and in the presence of various components in detergents and cleansing agents, sodium percarbonate has to be stabilized against the loss of active oxygen (O.sub.a). An essential principle for stabilization involves surrounding the sodium percarbonate particles with a coating of stabilizing components.
GB 174 891 has already disclosed spraying active oxygen-containing compounds with a sodium waterglass solution and then drying, in order to increase the storage stability. Sodium percarbonate is also stabilized by applying an adequate amount of silicate to sodium percarbonate particles, obtained by crystallization, in the process in DE-OS 26 52 776. A satisfactory stabilizing effect, especially in the presence of detergents and cleansing agents, is not produced by the previously mentioned methods.
Processes for stabilizing particulate sodium percarbonate are known from DE-OS 24 17 572 and DE-PS 26 22 610, wherein either a mixed compound which is formed by crystallizing a sodium carbonate with other inorganic salts such as sodium bicarbonate and/or sodium sulphate, or a mixture of sodium carbonate, sodium sulfate and a sodium silicate is used as a coating substance. In these processes, an aqueous solution of the constituents for the coating material is sprayed onto sodium percarbonate particles in a fluidized bed while maintaining a fluidized bed temperature of between 30 and 80.degree. C., wherein a solid coating is built up by evaporation of the water which is introduced. Despite the greatly improved stability of sodium percarbonate particles coated in this way, the active oxygen content still decreases too rapidly during long-term storage in the presence of a washing powder.
Numerous methods for the effective stabilization of sodium percarbonate by using boron compounds, such as boric acids (DE-PS 28 00 916), borates (DE-OS 33 21 082) and perborates (DE-PS 26 51 442 and DE-PS 28 10 379) are known. Despite a sometimes very good stabilizing effect, the market is increasingly interested in coated sodium percarbonate which does not contain boron compounds.
Another type of coating component is known from U.S. Pat. No. 4,325,933. According to this, stabilization is achieved by treating the sodium percarbonate with an aqueous solution of an alkaline earth metal salt, either magnesium sulfate or magnesium chloride. When treating the sodium percarbonate particles, a layer made of an alkaline earth carbonate is formed on the surface of the particles, which reduces the hygroscopic character of the sodium percarbonate and increases the stability. However, it has been shown that this type of stabilized product does not possess satisfactory storage stability.
EP-A 0 405 797 discloses sodium percarbonate compositions which are more reliable. According to one embodiment, the composition contains a compound from the group of inorganic magnesium compounds in addition to an alkali metal carbonate. The composition mentioned does not comprise sodium percarbonate particles with a uniform coating which adheres firmly to the sodium percarbonate core, but a mixture of substances which can also be granulated. During the finishing operations in the process given in the document mentioned, it was established that the storage stability of this type of composition, in the presence of a washing powder in a warm and moist environment, is not satisfactory; see comparison examples CE 2 to CE 4.
Accordingly, an object of the invention is to provide new, coated, sodium percarbonate particles with high storage stability in the presence of washing powders, wherein the coating should not contain boron compounds. The storage stability of the new coated sodium percarbonate particles is intended preferably to surpass that which has been obtainable using hitherto known, boron-free, coated sodium percarbonate particles. A further object of the invention relates to the provision of a suitable process for preparing storage-stable, boron-free, coated, sodium percarbonate particles.